1. Field of the Invention
The present invention is related to digital communication in uncoordinated radio systems and, in particular, to a method and apparatus for optimizing system performance by adapting the packet type to the conditions of the communication channel.
2. Description of the Related Art
In certain wireless communications systems, the data to be communicated are typically transmitted over a channel in bursts of packets. The packets may vary both in the length of each packet in the burst and the encoding of the packets. For example, in a high bit error rate environment, it may be advisable to use shorter length packets in order to reduce the number of corrupted packets received. Similarly, where the channel environment renders an error correction scheme ineffective, it may be advisable to switch to uncoded instead of coded packets. Moreover, channel conditions are subject to changes that may make a previously optimum packet type suddenly unsuitable for the new channel conditions. Therefore, it is desirable to be able to provide a way to select the packet type, and to do so dynamically based on the condition of the wireless channel.
Dynamic packet selection would be especially useful to reduce the effect of interference in uncoordinated or ad hoc networks like the Bluetooth™ wireless network that operates in the unlicensed 2.45 GHz, ISM (Industrial, Scientific and Medical) band. Referring now to FIG. 1, an ad hoc network 10 is shown comprising a number of electronic devices including a laptop computer 12, a personal digital assistant 14, and a mobile phone 16, all of which are interconnected by wireless links represented here by the jagged lines. The nature of the ad hoc network 10, as the name implies, is such that the network connections may be quickly added to and removed from the network 10. For example, additional mobile phones 16 may be easily added to the ad hoc network 10 simply by bringing the new mobile phones to within a certain range or distance of the network 10.
However, because the ad hoc network 10 operates in an unlicensed (unregulated) frequency band, other devices and networks that may be using the same band may unexpectedly interfere with the operation of the network. In fact, since it is possible for an interfering device to be much closer to the receiving device than the intended transmitting device, the signal-to-interference ratio (SIR) can be extremely small, on the order of −20 dB. Also, for a system operating in an unlicensed band such as the 2.45 GHz ISM band, the interferer may be a device with significantly higher output power, such as microwave ovens. In that case, the SIR can be up to 20 dB beyond the range where the network is working properly.
One method of reducing the effect of interference in an ad hoc network is to use some type of spread spectrum technique. There are essentially two ways that a spread spectrum system can mitigate interference: interference suppression and interference avoidance.
An example of interference suppression can be found in Direct Sequence Spread Spectrum (DSSS) where the information signal is spread over a wide band by the spreading sequence. Signals that are not spread according to the spreading sequence are suppressed at the receiver side. Here, the instantaneous bandwidth of the carrier greatly exceeds the bandwidth of the information to be communicated. However, the processing gain (PG), which is the ratio of the carrier bandwidth to the information bandwidth, is obtained through the spreading sequence. Therefore, for a multiple access system based on DSSS to work properly, the interfering signal should not be too much stronger than the desired one because suppression of the interferer will be limited by the PG. (See, e.g., J. C. Haartsen, “The Bluetooth Radio System,” IEEE Personal Communications, Vol. 7, No. 1, February 2000.)
A multiple access technique which is based on interference avoidance can be found in Frequency Hopped Spread Spectrum (FHSS) where the amount of time the information signal dwells on any carrier frequency is kept very short (less than 10 ms) to reduce the probability of being “hit” by an interferer. In FHSS, the instantaneous carrier bandwidth is on the same order as the information bandwidth (unlike in DSSS). Instead, the PG is obtained by letting the carrier frequency hop over a total bandwidth which is in great excess of the instantaneous bandwidth.
Although the above techniques may be effective in their own way, none provide a method or apparatus for selecting a packet type based on the channel conditions or changes thereto in order to minimize the effects of interference.